1. Field of the Invention
The present invention relates to a planographic printing plate precursor, and particularly, to a positive planographic printing plate precursor for so-called direct plate making that is capable of making a plate directly from digital signals of a computer and the like.
2. Description of the Related Art
Planographic printing is a printing method using a plate material having a lipophilic region that receives ink and an ink repellent region (hydrophilic region) that does not receive ink but receives dampening water. Recently, planographic printing plate precursors having photosensitivity (PS plates) are being widely used.
PS plates obtained by disposing a photosensitive layer on a support made of an aluminum plate or the like are being made practical, and are widely used. Printing is performed using these PS plates by removing the photosensitive layer at a non-image portion by image exposure and development, and using a hydrophilicity of a surface of a support and a lipophilicity of the photosensitive layer at an image portion. In the PS plate, being removable without generating a residual film is required of the non-image portion, while, excellent close adherence of the recording layer to the support without easily peeling is required of the image-portion. Further, in the non-image portion, the hydrophilic surface of the support is exposed after the removal of the recording layer by the development. However, when the surface of the support does not have a sufficient hydrophilicity, staining occurs in produced prints due to an adhesion of ink to the surface. Therefore, the surface of the support is required to have a high hydrophilicity from the standpoint of preventing the staining at the non-image portions.
Conventionally, as the hydrophilic support or the hydrophilic layer used by a planographic printing plate, generally an anodized aluminum support is used, or to further enhance hydrophilicity, a support obtained by silicate-treating this anodized aluminum support is used. Further, there are many studies on the hydrophilic layers and the hydrophilic supports that use these aluminum supports. For example, Japanese Patent Application Laid-Open (JP-A) No. 7-1853 discloses a substrate treated with an undercoating agent composed of polyvinylphosphonic acid, and JP-A No. 59-101651 discloses a technique of using a polymer having a sulfonate group as an undercoating layer for the photosensitive layer. Further, a technique of using a polyvinylbenzoic acid or the like as an undercoating agent has also been proposed.On the other hand, various techniques have been proposed relating to a hydrophilic layer which is utilized when a flexible support (substrate) such as PET (polyethylene terephthalate), cellulose acetate and the like is used instead of a metal support such as the aluminum support. Examples thereof include a swelling hydrophilic layer composed of a hydrophilic polymer and a hydrophobic polymer disclosed in JP-A No. 8-292558, a PET support having a micro-porous hydrophilic cross-linked silicate surface disclosed in EP No. 0,709,228, a hydrophilic layer which is hardened with a hydrolyzed tetraalkyl orthosilicate and contains a hydrophilic polymer described in JP-A Nos. 8-272087 and 8-507727, and the like.
These hydrophilic layers have enhanced hydrophilicity of the plate as compared with conventional plates and provide a planographic printing plate that obtains a print having no stains at the beginning of printing. However, these hydrophilic layers have problems wherein peeling is occurs during repeated printing and a decrease in hydrophilicity occurs with the passage of time. Therefore, there is a desire for a planographic printing plate precursor that obtains large number of prints having no stain, having a hydrophilic layer that does not peel from the substrate and in which the hydrophilicity of the surface does not decrease even under more severe printing conditions. From the standpoint of practicality, further improvement in hydrophilicity thereof is also required.
Development of lasers has been remarkable in recent years, and particularly, solid lasers and semiconductor lasers, which emit a light in a range of from near infrared to infrared wavelengths, have a high output and are portable, are easily available. As a light source for exposure for making a plate directly based on digital data of a computer or the like, these lasers are very useful.
In the recording layer of the positive type planographic printing plate precursor for use with an infrared laser, an alkali aqueous solution-soluble binder resin and a light to heat converting agent, such as an infrared absorbing dye and the like, which absorbs light and generates heat are contained as essential components. The light to heat converting agent acts as a dissolution inhibitor substantially decreasing the solubility of the binder resin by a mutual action with the binder resin in non-exposed parts (image portions). In exposed parts (non-image portions), the mutual action of the light to heat converting with the binder resin weakens by the generated heat, and causes the photosensitive layer to dissolve in an alkali developer. In this way, a planographic printing plate is formed.
However, though energy necessary for an image forming reaction is obtained on an irradiated surface of the precursor when it is exposed by the laser, the image forming property of a recording layer has a problem wherein thermal diffusion from the recording layer into a support is remarkable, particularly when a general-purpose aluminum support having an excellent heat conductivity is used as the support, and energy applied is not sufficiently utilized for image formation and sensitivity of the photosensitive layer is low.